1. Field of the Invention
The present invention relates to a method and apparatus for making a sand mold, more particularly to a molding method and apparatus, wherein the joint surface of the sand mold can easily be formed in such a way as to be projected or recessed from the edge of a molding flask in accordance with the shape of a casting product by the use of molding flasks in hand.
2. Description of the Prior Art
The parting plane of a pattern for making a sand mold is determined in consideration of the shape of a casting product. In general, each half of the pattern split along the parting plane is mounted on the surface of a pattern plate with the parting plane thereof, and a molding flask is placed on the pattern plate. An amount of molding sand is filled over the pattern in the molding flask, rammed by any proper means and then stripped off the pattern to form a mold cavity left behind the pattern. Thereafter, two halves of the sand mold or cope and drag are assembled to complete the sand mold. Thus, the parting plane of the pattern or the joint surface of the sand mold coincides with the lower edge of the molding flask.
For such a shape of the casting as shown in FIG. 6, however, an upper half section of the mold cavity in the cope and a lower half section of the mold cavity in the drag are unbalanced in their height, resulting in the difference of sand thickness from the respective section of the cavity to the edge of the respective flask to cause various problems when molten metal is cast into the sand mold. First, a small thickness of the molding sand causes an irregular packing density of sand to be easily deformed in portions of the sand mold with a low packing density to lead to dimensional inaccuracy or defects in the produced casting. Secondly, temperature remarkably rises in portions of the sand mold with a small thickness when molten metal is poured thereinto, so that the quality of the molding sand in those portions deteriorates to cause problems in reclamation of used molding sand.
According to the prior art to solve such problems as described above, there have been proposed several methods such as the use of tripled flasks or filling different amounts of sand into the upper half and lower half of the sand mold. These methods, however, respectively have disadvantages such as the need of complicated and large-size apparatus or difficulties in uniforming the packing density of the filled sand. Another method, as shown in FIG. 7, wherein the parting plane of the pattern and the corresponding edges of the flasks are positioned in a stepped manner and connected with a sloping surface with each other to form a joint surface of the mold, is capable of uniforming the thickness of sand on the pattern in the upper half and lower half of the mold, but the pattern is complicated in construction and heavy in weight and the productivity of castings may decrease.
Furthermore, since a demand for the casting products of a small lot recently tends to increase and their life cycle has been shortened, it has become very difficult to always establish molding conditions adequate to each of these products with the use of molding flasks in hand, causing a renewal of the sand molding line as a whole in extreme cases.